This Master Cylinder is Not usable! Survival Techniques When You Want to Keep Using It

When it comes to disc brake maintenance, we tend to focus on the brake pads and calipers, but the master cylinder, which applies the hydraulic pressure to the caliper, should not be neglected. If you take off a rear brake master that has been sitting for more than 10 years, you may find that the master piston is completely seized! Here's a technique that might help you get out of this situation. Here's a technique that might help.

If the master piston doesn't return, the brakes will stay on.

This is an example of a front master cylinder. When the snap ring at the end of the master cylinder is removed, the piston is pushed out by the tension of the spring at the rear.

When the brake lever is squeezed, brake fluid is pushed out of the master cylinder, and that volume of fluid is transferred to the caliper piston, which pushes the pads out and pinches the brake rotor. Without going to the trouble of explaining it, many riders will be familiar with the principle of disc brake operation.

The reason why the brakes release and become ineffective when your hand or foot is removed from the brake lever or pedal is that a spring set in the master cylinder pushes the piston back. This return spring is equipped in both drum brakes and bicycle rim brakes.

However, while the return springs for the drum and rim brakes are the outer type that can be seen from the outside, the master cylinder springs are the inner type, so you may not normally be aware of them.

Disc brake maintenance focuses on the caliper area, such as checking pad levels and cleaning and rubbing out the pistons, and you may not have the opportunity to focus on the master cylinder.

However, when the piston pushed in by the brake lever or brake pedal returns smoothly to its original position, the brake fluid that had been flowing into the caliper returns to the master cylinder, and the caliper piston retracts into the caliper, causing the pads to release from the brake rotor. Considering the brake release mechanism, it is very important that the piston moves smoothly inside the master cylinder.

Since it is the rider who operates the lever or pedal, he can push the piston. If the friction between the piston and the cylinder exceeds the tension of the spring, the piston may not return to its original position. This will result in constant fluid pressure from the master cylinder to the caliper, and the brakes may drag or lock up.

We have previously discussed the importance of lubricating the sliding pins in pin-slide calipers, but poor return of the master cylinder piston can cause brakes to drag, whether in pin-slide calipers or opposed piston calipers.

I'd like to replace the assembly for a master cylinder that won't release the piston, but...

After years of neglect, brake fluid leaked from the master cylinder, rusting the pushrods and sticking the master piston. the Kawasaki KZ900LTD was the only model with a master cylinder of this design, as the KZ900 of the same year had drum brakes.

When overhauling brakes, whether master cylinders or calipers, it is of the utmost importance to understand how they work, how they are constructed and to work with the correct components. It is often said that a motorcycle that won't stop is far more dangerous than one that won't work.

Here is an example of a reimported motorcycle that was manufactured 44 years ago, registered until around 2000, and not driven for the last 10 years or so. The front and rear brakes were sticking, but the rear brakes were particularly bad. If you stepped on the brake pedal even a little bit, the brakes would lock up and you had to push the pin slide-type caliper body to push the piston back each time. Of course, you can't drive.

The push rod between the brake pedal and the master cylinder is rusted red, which is probably due to the fluid leaking from the master cylinder through the push rod.

When I remove the master cylinder from the frame and remove the snap ring that holds the piston in place, the piston stays in the cylinder and shows no sign of coming out. Even if you pump air from a compressor through the banjo where the brake hose attaches, it will not budge. Depending on the design of the master cylinder itself, the piston can be pushed out from the hose banjo side, but this master cylinder cannot do that either.

Fortunately, genuine parts for the inner parts such as master piston, piston cup and spring are still supplied, but the master cylinder itself is no longer available. If the inner parts are still available today, it gives us hope that there might be some current models of master cylinders that use those inner parts, but we can't find any current models that use the parts for this model with the master and reserve tank in one piece.

If it is only to restore the function of the brake, I understand that it is realistic to install a master cylinder that can be used without question, but I cannot give up the feeling that I want to make the best use of the original because it is a conspicuous part in terms of design.

Does it work? Careful judgement is needed to identify the parts.

1) Remove the master cylinder and push rod from the vehicle body. The master cylinder piston is held in place by a snap ring, but the push rod is just stuck in the recess of the piston and can be easily removed.
2) Piston end with push rod removed. If the piston end was sticking out of the master cylinder like the front brake, you could grab it with a pair of pliers, but since it's inside the cylinder, you can't pick it up and pull it out. I removed the snap ring and turned on the compressor air from the banjo side, but it didn't budge.
3) I'm going to plan to use the recess where the push rod goes as a guide to tap and screw in the bolt and pull it out.
4) The tip of the piston broke off because the female thread was shallow and pried the bolt.

(5) The remaining part of the piston, which broke off at the tip, is covered with crystallized brake fluid. Fortunately, it was possible to drill and tap the hole, so I tried again. Be careful not to make the hole too big or you might damage the inside of the cylinder.
(6) Since there is a risk of prying again when pulling the handle through the bolt, the strategy was changed to a method of fixing the handle and pushing the body. Using a hydraulic press, the body is pushed through with slow pressure.
7) Luckily, I was able to push the piston out. The side of the piston was scratched by the hardened fluid, but the inside surface of the cylinder, which was cleaned with hot water, was not scratched, so it was determined that the piston could be reused. After restoration, it is necessary to check carefully for fluid leaks.
(8) I didn't know which model the master cylinder itself was diverted from, but the genuine manufacturer's parts were available for the piston, seal and spring. It was fortunate that the inner parts were available because the master cylinder could not be used if the seals could not be purchased even if the piston was removed.

I think more than half of the reason the piston is stuck in the master cylinder is because the brake fluid has crystallized between the piston and cylinder. Brake fluid is hygroscopic, so I tried soaking it in hot water and stroking the piston very slightly to see if that would melt the crystals and get it moving, but no luck.

So I made a tap on the end of the piston, where the push rod goes in, and screwed a bolt into it to pull it out. However, the clearance between the piston and the cylinder is very small to prevent brake fluid leakage, so the bolt screwed into the piston broke off easily just by prying a little.

When removing a broken bolt, a drill may be set up in the center of the bolt and the diameter may be gradually enlarged and shaved off, but if the inner surface of the cylinder is damaged, it is the end of the story.

Here I was able to remove the sticking master piston by pushing the master cylinder body instead of pulling the bolt. When I observed the removed piston, I could see that the crystallized brake fluid tightly adhered to the piston cup and piston body, and it was stuck because of this.

However, there was no rust on the inner surface of the aluminum alloy master cylinder, and when I installed the new inner parts such as the piston and spring, it stroked very smoothly and the brake fluid did not leak. The new inner parts such as piston and spring were installed and it strokes very smoothly and brake fluid does not leak.

It is unlikely that the master piston will stick on a motorcycle that is ridden regularly, but if the brakes on a motorcycle that has been immobile for a long time drag or lock, check the operation of the master cylinder piston as well as the caliper. If it is not working properly, pull the piston out and see if there is something wrong with the cylinder.

However, please understand that the cases introduced here are examples of successful cases, and not all pistons can be reused if the stuck piston is pulled out.

See KAWAWASAKI Moto Index Page
See Accessories for KAWASAKI Z900 (KZ900)
See Accessories for KAWASAKI Z900LTD (KZ900LTD)